Gatsby themes have come a long way in the last few months. We've iteratively shipped functionality that has enabled people to ship sites quickly on short deadline with an absolute minimum (1) of breaking changes. In this post we'll cover what we've shipped to date, where we are today, and what the roadmap looks like for where we're going:
- Theme Composition
- Component Shadowing 2a. Improving Docs
- Simplifying the Data Model
- ?
Theme Composition
The core theme composition algorithm was the first set of functionality we worked on. This piece of the code at its core can be thought of as Object.assign for gatsby-config.js. Given an array of themes, each gatsby-config is merged into the next. For example, if we have [themeA, themeB], the resulting config is { ...themeA, ...themeB }.
There are also some special considerations for how we merge specific fields of the gatsby-config. siteMetadata is merged using Lodash's deep merge, allowing users to override fields like the title of a blog, or the tokens in a design system that are shared across multiple themes. plugins are added together so that each theme's plugins are represented in the result ([...themeA.plugins, ...themeB.plugins]). To allow the usage of gatsby-* lifecycle APIs in themes, we also add each theme as a plugin itself, resulting in [...themeA.plugins, themeA, ...themeB.plugins, themeB]. This results in the theme being able to add functionality on top of any plugin it includes.
For more on this, read Introducing Gatsby Themes.
We introduced one major change to composition after the initial release to support child themes. A child theme is a theme that also uses the plugins gatsby-config key – a change that brought the full power of gatsby-config to theming.
Child themes under the hood are implemented by extending the original composition algorithm to be recursive. This means you can have as many or as few themes as you want, and that implementing it was not a breaking change for anyone. One really interesting facet of child themes in Gatsby compared to anywhere else is the ability to have a tree of themes. In the following example each theme points to its parents. (So gatsby-theme-supertheme references gatsby-theme-a and gatsby-theme-b in its gatsby-config). We have 7 total themes and the user only has to know about and install gatsby-theme-supertheme.
This flexibility allows us to come up with new patterns for abstracting functionality into themes. Since we're still in the early days we don't know what best practices will end up being and if you're just getting started its a good idea to build out a single theme before attempting to build a set of themes that work together.
Component Shadowing
The next problem we approached can be described as "How do I change the navigation component in a theme?".
Composition is useful but if you can't override the way data displays by changing the rendering, then we are left with fairly inflexible systems. Given that page templates and other functionality can be changed using Gatsby lifecycles such as onCreatePage, that left us with how to allow arbitrary changes to how a specific sub-section of the page renders. Component shadowing came next as the solution to this problem.
Component shadowing is based on the idea that Gatsby sites are built up out of React components. At any given point in the tree that is your site, you have a React component (or set of components) responsible for handling the rendering of some props. This could be low level like a Text or Heading component, higher level compositions like Media objects, the Navigation component, or even a full page layout. Component shadowing allows you to find the place a component is defined in a theme, such as gatsby-theme-blog/src/author.js, and replace it by creating a file in your site at my-site/src/gatsby-theme-blog/author.js.
In fact, any file that is processed by webpack can be shadowed.
To learn more about child themes and component shadowing, read this themes update blog post.
Improving Documentation
After composition and shadowing were released, we focused a bit on fixing any bugs that popped up and expanding usage to a wider audience. Some companies have already shipped child themes and component shadowing to production with impressive results: lowering development time and enabling development that might not have been able to ship within the necessary timeframe on other platforms.
We've also written and are updating a set of documentation that will be released on gatsbyjs.org for anyone that is using or looking to start using themes.
Simplifying the Data Model
We shipped composition, child themes, and component shadowing and we're not stopping there. Next up on our plate is something you may have seen us tweeting about: dealing with data. The GraphQL model Gatsby uses is fantastic for doing any kind of custom work, which is great for theme creators, but it means that as a theme user you're often locked into using MarkdownRemark when you'd rather be using Mdx.
Currently most of the folks using themes are experienced Gatsby users. As we start to have more and more themes publicly available, we'll have more and more newer Gatsby users installing and using them. Offering a simplified data model that can switch between backing content types means that instead of a theme tying itself to MarkdownRemark, it can tie itself to a generic data type like BlogPost that can be implemented by MarkdownRemark under the hood. We could use these generic data types as an (optional, extensible) target for theme creators. More creators building around community vetted core types means that more themes will be able to be swapped in place of one another and more themes will be compatible composing together. Imagine a blog theme that extends the core community BlogPost type with multi-author support. It would be immediately compatible with any theme that used the BlogPost type.
If you're following closely you may have already seen a blog post by Mikhail Novikov on the new schema customization APIs: createTypes and createResolvers. These are the primitives we are using to build data abstractions to use with themes. We're still experimenting and doing research here so I won't show any code today. If you want to participate in the development of the data abstraction, I highly encourage you to install gatsby@schema-customization and play around.
The Future
There are a number of directions we can go from here to give a better user experience to developing and using themes for experienced Gatsby contributors and new users alike. Building out potential UX improvements to shadowing for more complex multi-theme authoring use cases, defining patterns for sharing design tokens and data abstractions across themes, and building a cohesive set of themeable components for a set of official Gatsby themes are all directions you may see us together with the community work on in the future.
That's a peek inside where themes is going. Get started building your own themes today and let us know what you build! Be sure to check back in on the Gatsby blog this month as we have theming related posts from @jlengstorf, @4lpine, and @jxnblk coming shortly.